This version: 29 May 2020.

Summary
In Southwest Mongolia there is a form of Meriones gerbil that does not fit field guide descriptions of those in range. It has a bicoloured tail with a clean white under-tail sharply demarcated from the orange-brown dorsal half; as well as longer dark hairs on a tail tuft, and darker hairs dorsally on the tail (photos of two specimens shown below). It seems that field guide accounts and other references do not fully describe all colour variations. Field guides, for example, indicate that the only gerbil in the region with a white under-tail would be Tamarisk Gerbil (nearest records are 150 km to the west); however Meriones psammophilus can also have white under-tails (Olga Nanova, pers. comm.  2020 May 29). I have summarised below the taxonomy of the Meriones gerbils in Mongolia, with reference to local field guides as well as later papers by Nanova (2014) and Nanova et al (2020); and included notable tail descriptions where available.

Brief taxonomic overview of Meriones in Mongolia

Batsaikhan et al (2014) described three Meriones gerbils in Mongolia:
(a) Meriones tamariscinus (Tamarisk Gerbil).
(b) Meriones unguiculatus (Mongolian Gerbil)
(c) Meriones meridianus (Midday Gerbil); including M. m. psammophilus.

Nanova et al (2020) note that “Midday jird Meriones meridianus sensu lato is a widely distributed and highly variable species complex, whose taxonomy is still controversial despite several genetic and morphological studies”. Nanova (2014) and Nanova et al (2020) concluded (on morphometric and genetic data) that there are three clades of M. meridianus with interspecific level of differentiation that should be treated as distinct species:
c1) M. psammophilus (Milne-Edwards, 1871).
c2) M. meridianus (Pallas, 1773).
c3) M. penicilliger (Heptner, 1933).

M. psammophilus inhabits the Mongolian-Chinese part of the superspecies range;  but excluding the Mongolian Dzungaria region which is inhabited by M. meridianus (Nanova, 2014). Nanova et al (2020) note that the distribution ranges of all three species (psammophilus, meridianus and penicilliger) overlap in Dzungaria. The Mongolian Dzungaria includes the Great Gobi B SPA in the southwestern corner of Mongolia. M. penicilliger does not occur in Mongolia. Nanova et al (2020) noted that Meriones chengi (Wang, 1964; Cheng’s Gerbil) from Northwest China is likely a junior synonym of M. psammophilus (Ito et al, 2010; Nanova, 2014).

Following Nanova et al (2020) there are four forms of Meriones gerbils in Mongolia; as listed below with some range and field identification comments:

1 – Meriones tamariscinus (Tamarisk Gerbil): In Mongolia it is restricted to Mongolia Dzungaria (see location map at end of this post). Smith and Xie (2013, p141) show that its range continues to the southeast, within China but close to the Mongolia border. With respect to the Tamarisk Gerbil records shown in Smith and Xie (2013) it would not be unexpected (?), considering topography and habitat, for Tamarisk Gerbil to be found in parts of the Trans Gobi Altai area east of the Santamu Basin as marked on the map; this area being a low-lying area connected to the China range of Tamarisk Gerbil, with no apparent physical barriers.
2 – Meriones unguiculatus (Mongolian Gerbil): Parts of Mongolia excluding Dzungaria. The range map for Mongolian Gerbil in Batsaikhan et al (2014) excludes the Great Gobi A SPA region but Smith and Xie (2013) show records just south of the border. Mongolian Gerbil has a sharply bicoloured tail that is dark-greyish brown above, pale brown below; with black tuft at the end; and has greyish-black claws (Smith and Xie, 2013). Batsaikhan et al (2014) note that Mongolian Gerbil has dark claws; and the tail tip forms a dark brown or black tail tassle.
3a – Meriones meridianus / Meriones meridianus meridianus: referring to the form restricted to Mongolia Dzungaria. Descriptions in Batsaikhan et al (2014 p178) would refer primarily to M. psammophilus. Nanova (2014 p8) notes that “black tail brush is  … present in some individuals of M. meridianus” but does not mention this for M. psammophilus.
3b – Meriones psammophilus / Meriones meridianus psammophilus: its Mongolia range excludes Dzungaria. Batsaikhan et al (2014 p178) describes this taxon as having light yellow claws, a uniformly coloured tail and not having a well-developed tail tuft. Nanova (2014 p8) notes that “M. psammophilus is the most brightly coloured morph within the superspecies, with the presence of a rufous tint”. Nanova et al (2014 p8) stated that “no distinct geographical variation was found for M. psammophilus” (with reference to cranial measurements). Meriones psammophilus is, however, quite variable in fur coloration and can have white under-tails (Olga Nanova, pers. comm.  2020 May 29). 

Photos of gerbils with white under-tails
The two gerbils below were photographed in 2016 on a Mongolian Gobi trip with Jon Hall and Phil Telpher. I photographed three specimens at the two locations and Jon Hall had a photograph of one at the first location. These gerbils have distinctly bicoloured tails that are white below and not pale brown and do not have dark claws; which should distinguish them from Meriones unguiculatus (Mongolian Gerbil). The photographed forms appear to represent a colour variation of Meriones psammophilus / Meriones meridianus psammophilus described by Nanova above.

Gerbil #1 was photographed at Mother Mountain (Eej Khairkhan Uul). Key features are (a) a clearly bicoloured tail that is distinctly white below; and orange-brown above with a darker brown dorsal section; the border between the white and orange is sharp; (b) tail tuft is orange-brown with longer black hairs dorsally and off the tip and more similar to that of Mongolian Gerbil (?) (c) hind claws whitish; (d) forefoot claws greyish.

Gerbil #2 was photographed in the Great Gobi A SPA; it has a tail very similar to that of Gerbil #1; both hindfoot claws appear whitish; fore-foot claws appear to be pinkish-grey to whitish. It was photographed at 8.50 am under a “gooseberry type/looking bush”.

Location Map
The locations of Gerbil #1 and #2 are marked as Boxes 1 and 2; they are 220 km apart. The Red Line shows the northern range limit in China and Mongolia of Tamarisk Gerbil (TG); data based on Batsaikhan et al (2014), Smith and Xie (2013 p141) and Tsytsulina et al (2016, IUCN Red List). It seems possible that the range of TG might extend eastwards from the Santamu Basin (as marked). The basemap data is from OpenStreetMap at https://en-gb.topographic-map.com/maps/lp6g/China/; cartography is licensed as CC BY-SA (this copyright page)

References

Batsaikhan N, Samiya R, Shar S, Lkhagvasuren D, King SRB (2014). A Field Guide to the Mammals of Mongolia (Second Edition). Zoological Society of London.

Nanova O (2014). Geographical variation in the cranial measurements of the midday jird Meriones meridianus (Rodentia: Muridae) and its taxonomic implications. Journal of Zoological Systematics and Evolutionary Research, 52(1):75–85. doi:10.1111/jzs.12032.

Nanova OG, Lebedev VS, Matrosova VA, Adiya A, Undrakhbayar E, Surov AV, Shenbrot GI (2020). Phylogeography, phylogeny, and taxonomical revision of the Midday jird (Meriones meridianus) species complex from Dzungaria.
Journal of Zoological Systematics and Evolutionary Research 2020:00:1–24. https ://doi.org/10.1111/jzs.12372.

Smith AT, Yan Xie (eds) (2013). Mammals of China. Princeton University Press.

Tsytsulina K, Formozov N, Sheftel B (2016). Meriones tamariscinus. The IUCN Red List of Threatened Species 2016: e.T13169A115110536. http://dx.doi.org/10.2305/IUCN.UK.2016-3.RLTS.T13169A22432783.en, Downloaded 2019 Oct 1.